LTE (Long Term Evolution) has been under study heretofore, for the purposes of achieving improved spectral efficiency and peak data rates, reducing delays and so on in UMTS (Universal Mobile Telecommunications System) (non-patent literature 1). As a result of this, in Release-8 LTE (hereinafter referred to as “Rel. 8-LTE”), as radio access schemes, a scheme that is based on orthogonal frequency division multiplexing access (OFDMA) was employed for the downlink, and a scheme that is based on single-carrier frequency division multiple access (SC-FDMA) was employed for the uplink. In Rel.8-LTE, it is possible to achieve transmission rates of approximately maximum 300 Mbps on the downlink and 75 Mbps on the uplink, by using a variable band that ranges from 1.4 MHz to 20 MHz. Presently, in 3GPP, successor systems of LTE (referred to as “LTE advanced” (“LTE-A”)) are under study for the purpose of achieving further broadbandization and faster speed beyond the UMTS network.
Recently, a study is progress to achieve increased network capacity by building a heterogeneous network (HetNet), in which low-power nodes (LPN) of low transmission power are overlaid in the area of a macro cell, and applying carrier aggregation (CA) to the HetNet. Carrier aggregation refers to the technique of achieving broadbandization by using a frequency band (1.4 MHz to 20 MHz) that is supported in LTE as one component carrier (CC) and using multiple CCs at the same time. In the HetNet, it is possible to realize efficient user terminal control, traffic off-loading and so on, by changing the connecting cell to which a user terminal is connected, on a per CC basis.
FIG. 1 shows, as an example, a state in which a user terminal UE is connected with two cells of a base station apparatus eNB (macro cell) and a low power node LPN (low power cell) in a HetNet. The user terminal UE is allocated component carriers CC #1 and CC #2 by carrier aggregation, and connects with the macro cell via component carrier CC #1 and connects with the low power cell via component carrier CC #2. Since the low power node LPN 2 has a small cell, the user terminal UE is located in a position closer to the low power node LPN than to the base station apparatus eNB. In Rel. 11-LTE, which is the latest standard of LTE-A, an MTA (Multiple Timing Advance) function to make it possible to define a plurality of transmission times for a plurality of CCs on the uplink is introduced (up to Rel. 10, a user terminal is subject to single-transmission time control (which is referred to as “TA” or “single TA”)), for the purpose of coordinating the times of reception between separate nodes (base station apparatus, low power node and so on). In the example shown in FIG. 1, the macro cell carries out uplink transmission at a transmission time T1, and the low power cell carries out uplink transmission at a transmission time T2, which is a predetermined time delayed from transmission time T1.
In LTE-A, carrier aggregation to use maximum five CCs is realized. In MTA, which is introduced in Rel. 11-LTE, maximum five CC are classified into maximum four TA groups (TAGs), and the times of transmission are controlled on a per TAG basis.
As an example, FIG. 2 shows a state in which five CCs are classified into four TAGs. Five of CC #1 to CC #5 are classified into four of TAG #1 to TAG #4. TAG #1 is assigned to CC #1, one TAG #2 is assigned to two of CC #2 and CC #3, TAG #3 is assigned to CC #4, and TAG #4 is assigned to CC #5.
When the times of uplink transmission are controlled on a per TAG basis in a user terminal UE where MTA is applied, as shown in FIG. 3, the difference between the transmission times of the TAGs may develop to approximately 30 μs at a maximum. FIG. 3 shows a state in which the transmission times of TAG #1 and TAG #2 are, for example, 30 μs different.